The enteric nervous system is a unique region of the peripheral nervous system that is particularly well suited for studies of the development of neuronal diversity. It has a structure and chemistry that resembles the CNS but is a much less complex system. The enteric nervous system has a limited number of known types of neuron (including, intrinsic cholinergic neurons, serotonergic neurons and neurons that store peptides as well as the axons of extrinsic adrenergic neurons) and their transmitter mechanisms provide an array of convenient markers for neuronal detection. Preliminary studies have indicated that a sequential order is followed in the appearance of enteric neurons that is reproduced in several mammalian species and also in birds. This observation, and the recent evidence that the microenvironment of the tissue in which development occurs is vital to the final determination of such neuronal characteristics as transmitter choice, has led us to want to test the hypotheses that the microenvironment of the gut is critical in enteric neuronal development and to the development of diversity among enteric neurons. There will be 3 parts to the proposed study. First, normal development of the enteric nervous system will be examined, utilizing transmitter-specific markers in order to obtain clearly defined quantitative and points against which the effects of experimental procedures can be judged. These studies will also use organotypic tissue culture techniques (in which neuraxial origins of precursors will be evaluated) and quantitative electron microscopy. Second, the lethal spotted mutant mouse will be studied as a model in which a segment of the gut is congenitally aganglionic. Finally, manipulations of the enteric microenvironment will be attempted.